Analysis of previous thin section and freeze-fracture observations of the tight junction (Zonula occludens) lead us to propose that the junctional strands are comprised of cylindrical, intramembranes inverted micelles. We propose that the exoplasmic halves of plasma membranes are fused into single leaflets that assure the semipermeability properties of the junction. We show that in various epithelia massive, rapid proliferation of tight junction strands is easily accomplished. In addition, we use toad bladder, a system amenable to physiological monitoring of transport functions, where we demonstrate that proliferation of junctional strands occurs after an osmotic shock. We studied the time course of this proliferation, the effect of cytoskeletal pertubers and the role of calcium ions. Application of morphometric techniques and statistical analysis provides time courses for assembly and also disassembly of the junctional strands.